1. Field of the Invention
The present invention relates to an illumination system and a display device using the same and more particularly, to an illumination system comprising Light-Emitting Diodes (LEDs) as point light sources, and a display device such as a Liquid-Crystal Display (LCD) device using the illumination system. The invention is preferably applied to a backlight unit of a LCD device; however, it is applicable to any other display device than the LCD device if it comprises plural sets of LEDs whose colors of emitted light are different.
2. Description of the Related Art
The LCD device has been extensively used for various devices and systems, such as Office Automation (OA) apparatuses, Audio-Visual (AV) apparatuses, portable information or communication terminals and so on, because of its characters such as compactness, thinness, and low power-consumption. The LCD device comprises as its main components a LCD panel having a pair of opposing transparent substrates and a liquid crystal layer interposed between the substrates, and a backlight unit for generating backlight illuminating the panel. The liquid crystal layer is controlled in such a way as to adjust the transmittance or transmitting quantity of light emitted from the backlight unit, thereby displaying images on the screen.
Conventionally, the backlight unit comprises a fluorescent lamp or lamps as its light source. However, to drive or turn on the lamp or lamps, peripheral circuits such as an inverter circuit are required. Moreover, the inverter circuit makes noises. Therefore, recently, it is popular to use LEDs instead of the fluorescent lamp or lamps.
Typically, the backlight unit comprises plural sets of LEDs arranged in a predetermined manner, where each set of LEDs emit red (R), green (G) or blue (B) light. The red, green and blue light emitted from the respective LEDs are then mixed together to generate white light. Thus, there is a problem that color irregularity is likely to occur on the display screen due to the arrangement or layout of the LEDs occurs.
To cope with this problem, an improvement was developed and disclosed in the Japanese Non-Examined Patent Publication No. 2000-36209. This improvement relates to a linear light source as shown in FIG. 1. As seen from FIG. 1, this prior-art linear light source comprises a set of LEDs 110a serving as point light sources 110, a diffusing plate 111, and a light guide plate 112. The diffusing plate 111 receives the light emitted from the LEDs 110a. The light guide plate 112 receives the light from the diffusing plate 111 and emits light from a flat surface of the plate 112. The LEDs 110a are arranged along a straight line at equal intervals P in parallel to a diffusing plate 111, as shown in FIG. 1A. The LEDs 110a are apart from the diffusing plate 111 at a distance L, as shown in FIG. 1B. The interval P and the distance L have a predetermined relationship, thereby generating approximately uniform luminance distribution.
In the case where a light source is formed by plural sets of LEDs whose light colors are different from each other, it is important to uniformly mix the light of the respective colors. With the improvement shown in FIGS. 1A and 1B where the distance L between the LEDs 10a and the diffusing plate 111 is increased, there is a disadvantage that the illumination system will be large. This enlarged size of the system will be a serious drawback if this improvement is applied to an illumination system of portable information or communication terminals. Moreover, there is another disadvantage that obtainable luminance is lowered due to the large distance L.
To reduce the color irregularity due to the arrangement or layout of the LEDs, there is a way of separately controlling the respective LEDs. In this case, however, many power supplies and many control circuits are required for lighting the LEDs. In particular, with an illumination system for illuminating a large-sized screen, the count of LEDs is large and thus, there arises a problem that the structure is complicated and the price is raised.